The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosus. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosus. The laminated nature of the annulus fibrosus provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contained sulfated functional groups that retain water, thereby providing the nucleus pulposus within its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines as well as matrix metalloproteinases (“MMPs”). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of disc degeneration disease (DDD), gradual degeneration of the intervetebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors, such as programmed cell death, or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of toxins.
As DDD progresses, the toxic levels of the cytokines present in the nucleus pulposus begin to degrade the extracellular matrix (in particular, the MMPs (under mediation by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing its water-retaining capabilities). This degradation leads to a less flexible nucleus pulposus, and so changes the load pattern within the disc, thereby possibly causing delamination of the annulus fibrosus. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
Intervertebral disc degeneration causes a number of clinical problems, including sequelae related to reduced disc height and herniation. In many cases, a simple discectomy can effectively relieve pain, but in time results in further collapse of the disc space because the intervertebral disc can no longer resist body loads the same as a healthy disc. Spine fusion procedures represent another state of the art treatment for disc problems. Fusion generally involves the use of interbody fusion cages and spinal fixation systems to immobilize the fusion site.
In an effort to substantially maintain the patient's range of motion and to reduce tissue damage associated with surgical intervention, the art has considered nucleus pulposus replacement and enhancement devices. Many of these devices are designed to fill at least a portion of the void left by removal of the nucleus pulposus portion of the disc and provide joint flexibility and shock absorption. Some of the nucleus pulposus devices being evaluated are in situ cured (such as in situ cured polyurethane contained within an outer bladder and in situ cured protein polymers). Other devices under evaluation include relatively solid hydrogels (such as hydrogel contained within a UHMWPE pillow and hydrogel balls).
Other intervertebral motion devices include devices having an articulation interface and cushion-type devices.
Both the fusion and motion intradiscal implants require an accurate determination of the cleared disc space for the best performance, mechanical fit and material interdigitation of the device in order to minimize potential device movement and expulsion.
Each of the above-noted treatments involving an implant requires a removal of the natural nucleus pulposus from the disc space. This procedure is called a “discectomy”.
The ability of a surgeon to accurately determine the position, size and shape of the cleared disc space during discectomy is currently limited by many factors, including the procedure approach, access, location and the size through the annular wall, as well as available intraoperative imaging techniques. Improper location, size or shape of the cleared disc space following discectomy can greatly impact the size, placement and securement of intervertebral devices that are ultimately placed in the disc space, as well as the biomechanical loading of the device and the physiologic response to the device. For example, improper lateral placement of a nucleus pulposus replacement device may cause migration or expulsion of this implant, leading to continued height loss and irritation of neighboring tissues (including nerve roots), thereby creating additional pain or requiring re-operation.
U.S. Pat. No. 5,888,220 (“Felt I”) discloses a nucleus pulposus replacement device comprising an expandable bag into which in-situ curable polyurethane is injected. Felt further discloses that the placement of the bag can be radiographically verified with the use of a C-arm. See also U.S. Pat. No. 6,248,131, US Published Patent Application Nos. US 2003/0220649 (“Felt II”) and US 2003/0195628. Felt II discloses some embodiments in which the balloon has metallic wires or other imageable means incorporated into it so that the balloon can be seen under fluoroscopy. Felt discloses that potential imageable materials include any metal, metal alloys, or ceramics that could be combined with a polymer, and that the imageable material can be in the form of wires, a mesh, or particles incorporated into the balloon or on its surface.
Felt does not disclose the use of a radiographic disc space trial balloon that is inflated to verify the size and geometry of the disc space.